A technology for protecting a rider on a motorcycle by mounting an air bag apparatus on a motorcycle is known. For example, Japanese Unexamined Patent Application Publication No. 2001-219885 discloses a scooter-type motorcycle and a technology for expanding and inflating an air bag located in the space between a body component member, such as a head pipe, and the seat on which the rider sits. The technology suggests the possibility of providing the air bag apparatus on the scooter-type motorcycle; however, further technological study on how much to expand the air bag inside the air bag apparatus generally provided on a motorcycle to protect the rider is necessary.